The first step is to separate the number 15625 into groups of three from right to left, so:

15 625

Now take the number 15 and compare it to the memorized chart of cubes. You will see that the largest cube which goes into 15 if 2³. So the first digit of your answer is a 2. Finally, take the other grouping, 625, and compare it's last digit, 5, to the last digits of the chart. The only option is 125, which is the product 5³. So the answer is 25!